1. Technical Field
This invention relates to an optical recording medium capable of writing and reading operation at a wavelength of up to 690 nm and having a recording layer and preferably a reflective layer, wherein pits are formed by irradiating a laser beam.
2. Background Art
The inventors have been engaged in the development of write-once compact discs (CD-R) as an optical recording medium capable of recording in conformity with the compact disc (CD) standard. The CD-R has on a substrate provided with a guide groove, a light recording layer of organic dye compound, a light reflecting layer of metal, and a protective layer of UV-cured resin and is capable of writing and reading operation with a laser beam of 780 nm wavelength.
In the recent years, there is a desire to have a higher density recording medium. It is a write-once digital video disc (DVD-R) which is arrived at by reducing the writing wavelength to 635 nm and the reading wavelength to 650 nm from the writing/reading wavelength of the current CD-R. To comply with this wavelength change, the organic dye compound used in the recording layer must be altered. One candidate is cyanine dyes, which undergo substantial light degradation and lack stability. Another candidate is azo compounds as described in JP-A 8-108623, JP-A 8-99467, etc., which are less resistant to light. Then, as dyes having high light resistance, JP-B 7-51682, JP-A 3-268994, and JP-A 8-156408 disclose metal azo complexes. Despite light resistance, these dyes have a low recording sensitivity and low solubility and suffer from the problem that when used in optical recording media, disc characteristics, especially reflectance (Rtop) and modulation remain unbalanced due to an increased half-value width of their absorption spectrum.
It is noted that JP-A 10-97732 describes an optical recording medium comprising a recording layer containing a specific xanthene dye such as Rhodamine B, and discloses an example in which the xanthene dye is used together with an azo metal complex. As to the combination of groups attached to opposite ends of the diazo group in the azo compounds which become the ligand of the azo metal complexes disclosed therein, a benzene or naphthalene ring combined with a pyrazole ring, and a benzene or naphthalene ring combined with an arylcarbamoyl-substituted alkene are allegedly preferable. In the Examples disclosed therein, the compounds used as the ligand are those compounds wherein, when represented by formula (I) as used herein, R.sub.8 is a sulfamoyl group, and R.sub.3 and R.sub.4 form a benzene ring, but the amino moiety is eliminated. Also disclosed is an optical recording medium using in a recording layer a dye formed from such an azo cobalt complex anion and a xanthene dye cation. However, even these examples of using the xanthene dye in combination with the azo metal complex have the drawback that recording fails due to sensitivity shortage or even when recording is possible, the reflectance and modulation at the reading wavelength are generally reduced because the azo metal complex has a short absorption wavelength.
Further, the inventors previously proposed azo metal complexes in WO 98/29257. They are the compounds wherein, when represented by formula (I) as used herein, both R.sub.1 and R.sub.2 are alkyl groups, and preferably R.sub.7 or R.sub.8 is a nitro group. Regrettably, these azo metal complexes were also found to have the drawback that high-speed recording is impossible because of somewhat low sensitivity although normal recording is possible.
Additionally, JP-A 2-55189 discloses an optical recording medium having a recording layer formed from a diol six-coordinate metal complex salt compound between a cyanine dye and a naphthalenino-azobenzene. Furthermore, JP-A 3-51182 discloses an optical recording medium having a recording layer containing a photo-stabilized organic dye in the form of a combination of an anion of an azo metal complex salt compound with a cation of a cyanine dye having absorption in the wavelength region of recording light. These optical recording media, however, conform to a wavelength of about 780 nm, but not shorter wavelength recording.